1. Field of the Invention
The present invention relates to a photographic lens system provided in a photographic camera (especially an SLR digital camera) or a video camera, and more specifically relates to a wide-angle lens system which can adequately correct aberrations while attaining a sufficiently long back focal distance.
2. Description of Related Art
In photographic cameras, electronic still cameras, and video cameras, etc., where a wide-angle lens system by which the focal length at the short focal length extremity can be shorter is employed, a retrofocus lens system in which a lens group having a negative refractive power (hereinafter, a negative lens group) and a lens group having a positive refractive power (hereinafter, a positive lens group) are provided in this order form the object is often used. This is because in the above cameras, there is a need to secure a predetermined distance between the most image-side lens surface and the imaging plane (i.e., the back focal distance).
Examples of related art wide-angle lens systems are taught in Japanese Unexamined Patent Publication Nos. H09-113798, 2005-316014, and 2006-301416.
A retrofocus lens system has an asymmetric lens arrangement with respect to the diaphragm, since a strongly negative lens group is provided in front (on the object side) of a master lens. Consequently, the correcting of aberrations, such as distortion and lateral chromatic aberration, occurred in the front lens group is difficult.
Specifically, in a lens system disclosed in Japanese Unexamined Patent Publication No. H09-113798, a high optical quality is achieved with respect to (the correcting of) aberrations; however, since refractive power is not suitably distributed to the most object-side lens group, the back focal distance cannot sufficiently be secured.
In a lens system disclosed in Japanese Unexamined Patent Publication No. 2005-316014, a glass material is not adequately selected, so that a large amount of flare caused by off-axis aberrations occurs; and refractive power is not suitably distributed over the lens groups, so that the entire length of the lens system disadvantageously becomes longer.
In a lens system disclosed in Japanese Unexamined Patent Publication No. 2006-301416, refractive power is not adequately distributed over the two lens groups on the object side from the diaphragm, so that off-axis aberrations, such as distortion and field curvature, largely occur. Furthermore, a glass material selected for lens elements to be cemented is inappropriate, so that lateral chromatic aberration and field curvature largely occur over the entire lens system.